The testers known currently on the market are fixed testers, which are blocked and rotate with the same rotational speed as the grinding wheel even when in contact with the stone to be shaped. These testers suffer from wear resulting from the hammering impacts of the stone. Rotating testers (which rotate with the wheel) come in contact with the stone so as to be arrested, braking and rubbing against the material itself, while the grinding wheel goes on working. The disadvantage is that, owing to the inertia of the outer part of the tester, burns can form on the parts of the stone that first come into contact with the tester, with consequent deformation and wear of the tester itself. A spacer plate tester is used which is not in contact with the grinding wheel, but is inserted free in a groove, through an approach system mounted on the machine, since said plate is calibrated to strike against the stone to be worked. The difficulty with this system. is that, besides being more expensive than other types, in order to have a perfect shaping, the grinding machine must be positioned perfectly perpendicular to the surface to be worked, otherwise waves and trenches can be produced in the surface itself, depending on how the machine is positioned relative to the stone to be worked.